1. Field of the Invention
The present invention relates to a digital color printer.
2. Description of the Prior Art
Conventionally, any digital color printer consists of image reading means and printing means. Using an image sensor like a charge coupled device (CCD), an image reading means reads the image of the original document and it converts the readout image into the digital values which can be subsequently converted into pring output colors which are yellow, magenta, cyan, and black. Printing means prints an image corresponding to those print output signals on a printing paper via an electrophotographic process. When printing full colors, the identical printing paper is repeatedly fed back to the printing process in order that an image for each color is printed on the identical printing paper in full shades.
In any conventional full-color printer, if the printer uses a buffer memory having an insufficient storage capacity instead of a large-capacity image memory which corresponds to the size of an original document than, in order to prevent the printing position from any deviation, the image reading operation must be executed synchronous with the printing operation. To achieve this mode of operation, an extremely fine accuracy is essential for exactly matching the printing position.
Assuming that the scanner, including light source 12 reads the top edge of the original document (see FIG. 2) at the moment when reference point A (like the top edge of the printing paper for example) of image-transfer drum 51 is at position B as shown in FIG. 1, where the image-transfer drum 51 transfers toner image onto the printing paper being wound on it, then an initial scanning and printing of the first color are executed subsequent to the scanning operation of the scanner and the rotation of image-transfer drum 51. Subsequently, the second scanning operation and the printing of the second color are executed. In FIGS. 1 and 2, if the scanner including light source 12 were in a condition to read point Q which is different from the top edge P of the original document when reference point A is at position B, the distance between P and Q is printed in the form of deviated color. This is because the scanner repeats the reciprocating movements, and thus, the shifting speed of the scanner right after starting the image-reading operation is unstable while the continuous rotation speed of the image-transfer drum 51 is stabilized. The compensate for this, any conventional digital color printer tries to constrain color deviation by precisely adjusting the rise characteristic of the scanner driving motor.
According to a prior art, device proposed by Japanese Patent Application Laid-Open No. 61-123257 (1986), the color image processing system is provided an image-reading operation which is preliminarily executed by delaying the image formation so that the buffer memory can absorb the difference. In other words, data of the first line of the original document is written into the buffer memory before the reading image. This prior art allows the start of the data reading from the first line and subsequently activation of the data writing operation. Nevertheless. since the data writing and reading operations are executed for each line, an address line must strictly be controlled. So a difficulty exists in that this requires complex control circuits.